Just like knotted-up headphones: Entangled photons stay entwined over record distance

Literally spooky action at a distance

Pairs of entangled photons created on a satellite orbiting Earth have survived the long, perilous trip from space to ground stations. Crucially, they are still linked despite being picked up by receivers over 1,200km (745mi) apart – the longest link ever seen before.

“This is a scientific breakthrough,” says Rupert Ursin, a quantum physicist at the Institute for Quantum Optics and Quantum Information in Vienna, Austria, who was not involved in the research.

Many teams around the world are duking it out to create secure quantum communication technology. Unlike securing messages from prying eyes with classical encryption, securing with quantum methods means any tinkering would leave a trace.

One idea is to send linked, or “entangled,” photons whose behavior changes when you try to tinker. The special “polarization” property (you could think of the direction of a bar magnet) of either correlated photon could act like both a secure encryption and decryption key.

Previously, researchers have been able to teleport entangled photons that remained correlated at distances of around 100km – demonstrating “spooky action at a distance,” as Einstein put it. For example, in 2012 researchers transported entangled photons about 146km apart from one another in the Canary Islands. The problem is that if you try sending quantum bits through the air or through fiber cables, losses are high, so the maximum distance for still being able to measure a correlation between photons has been limited, Ursin says.

In the new study, Chinese researchers used the custom-built “Micius” satellite at an orbit of approximately 500km to create six million entangled photons and blast them at ground stations in China that were continuously checking for matching photon pairs. The magic is that there’s less signal loss if you distribute the paired photons through space via satellite.

Chao-Yang Lu, a quantum physicist at the University of Science and Technology in China who worked on the data analysis for the project, says it was difficult to pull the experiment off because of diffraction as well as absorption and turbulence in the atmosphere. Aiming is also a challenge because of the high speeds of the satellite and its distance to the ground.

How are correlated photons created?

The two correlated photons are created when a laser shines through a crystal. Eventually, by verifying a correlation test between two photons known as Bell’s inequality (if two photons are correlated, they violate it), the team discovered that the ground stations – separated by 1,203km – could detect a single pair of correlated photons every second.

“The data rate is still low,” Ursin says. If you wanted to encrypt a 5,000-bit email message using this proof of concept experiment, that would take 5,000 seconds. But it’s still a big step forward for the field, he says.

Christoph Marquardt, a quantum physicist at the Max Planck Institute for the Science of Light in Erlangen, Germany, says: “It’s kind of surprising how well it worked,” given the experimental conditions.

But he still thinks it’s decades off from practical application for encryption. One of his latest papers on quantum communication, which appears in Optica, shows that if you were willing to trust a third party to store your quantum keys (instead of having that third party have no knowledge in the entangled photon scenario), you could measure "quantum states" on satellites 38,000km away in space. He believes that this sort of quantum communication is much closer to practicality.

Still, Lu says he wasn’t too worried about the practical applications just yet. At some level he says he would have been alright if the experiment didn’t work and the team would discover new physics. He says a next step is trying to make the satellite work during the day – the team ran the experiment around midnight to limit the noise from stray light. He added they also hope to explore higher orbits.